1. Field of the Invention
This invention relates to a TiAl based intermetallic compound alloy comprising .gamma. and .beta. phases having a supermicrostructure and a process for producing same.
2. Description of the Related Art
Among intermetallic compounds, many compounds have specific properties which a single phase metal does not possess and there have been investigated for application as functional and/or constructional materials. For example, since Ni.sub.3 Al, TiAl and the like have a strong positive temperature-dependency of strength, they have been increasingly expected to be applied as heat-resistant materials. In particular, TiAl, which has a low density of 3.8 g/cm.sup.3, has been investigated for application to aircraft materials. Most of the intermetallic compounds including TiAl have a poorer deformability than general metals, and thus many investigations into an improving of their ductilities have been made.
Concerning the TiAl based intermetallic compounds, techniques wherein Cr is added as the third element for improving the ductility are disclosed in U.S. Pat. No. 4,842,819, Japanese Unexamined Patent Publication (Kokai) No. 64-42539, Japanese Unexamined Patent Publication (Kokai) No. 1-259139, etc., but these are all intended only for a grain refining by the addition of Cr.
In addition to the alloy design by alloying, an attempt to control the microstructure by a thermomechanical treatment has been made to thus improve the deformability. For example, isothermal forging process for TiAl binary alloy has been disclosed (Japanese Unexamined Patent Publication (Kokai) No. 63-171862.) Through isothermal forging, equiaxed grains having 10-20 .mu.m diameter were obtained. Although these microstructure controlled samples have a high deformation stress at 800.degree. C., the room temperature ductility was not improved. Further, it has been reported that an intermetallic compound Ti-33.5% Al-2% Mo-0.05% B-0.09% O in weight was thermomechanically treated (hot-extrusion followed by isothermal forging) for grain refinement and the mechanical properties at high temperature were examined, which showed a superplastic deformation behavior exceeding 80% tensile elongation at 800.degree. C. (Abstract of Autumn Symposium of The Japan Institute of Metals (1989), pp.238). Nobuki et al., reported that the microstructure controlled by isothermal forging samples, having a 13 .mu.m grain, which composition was Ti-35% Al in weight, showed a higher m value (strain rate sensitivity factor) over 0.3 and had a high temperature strength. Further, it was reported that, when the temperature was controlled within the range of 887.degree. -1047.degree. C., repeated sudden temperature change at a strain rate of 10.sup.-3 S.sup.-1, allowed a 220% fracture point to be obtained (Abstract of Autumn Symposium of The Japan Institute of Metals (1989), pp.245).
Further, the technique wherein a TiAl based intermetallic compound alloyed with Mo as the third element is isothermal forged to precipitate a .beta. phase in the .gamma.-grains, was reported in the Material of 53th Meeting of Superplasticity (Jan. 30, 1990, pp. 1-5). According to this report, the compound had an m value higher than 0.3 only in the case of a strain rate lower than 5.times.10.sup.-4 sec.sup.-1 at 1273 K, and the best value was 230% .
It is well known that a TiAl based intermetallic compound alloy has a low ductility at room temperature, and does not possess a good workability even at high temperatures, in comparison with that of usual alloys. As disclosed in Abstract of Autumn Symposium of The Japan Institute of Metals (1989), page 245, one of the above-mentioned references, even if such special heating-cooling treatments are applied with repeated sudden temperature variations in the range of between 887.degree. C. and 1047.degree. C., at a fixed strain rate the 10.sup.-3 S.sup.-1 is 220% at most. Furthermore, according to the report of the Material of the 53th Meeting of Superplasticity, the optimum data for a tensile elongation tested at 1273 K (about 1000.degree. C.) at a strain rate lower than 5.times.10.sup.-4 .sup.-1 (the report did not clearly show the strain rate, but generally the lower the strain rate the greater the elongation at fracture.) was as low as 230% .
As described above, since a TiAl based intermetallic compound has characteristics such as a light weight, good heat resistance and high strength, the application thereof, for example, to the material forming the main parts of supersonic airplanes and spacecraft in the space fields, and automotive parts such as the valve material for automobile engines and turbocharger rotors, has been expected, and there is a need to further improve the workability.
An object of this invention is to provide a novel TiAl based alloy having a high fracture elongation and an m value which cannot be obtained by the prior art technique and a process for producing the same.
Another object of this invention is to provide a TiAl based alloy having an enhanced yield strength inherent to the TiAl based alloy.